The (8;21) translocation, reported in 40% of M2 subtype acute myeloid leukemia (AML), is the second-most-frequently observed example of a nonrandom genetic alteration associated with AML. Juxtaposition of the AML1 gene on chromosome 21 to the ETO gene on chromosome 8 fuses the N-terminal portion of AML1 to near-full length ETO, creating AML1/ETO. Previously, work has focused on perturbation of AML1 gene function by the chimeric fusion protein. The AML1-CBFbeta heterodimeric transcription factor complex is a frequent target of rearrangement. The AML1/ETO fusion may suppress wild-type AML1 function by competition for DNA binding or heterodimer formation with CBFbeta. In contrast to AML1, less is known about ETO. ETO has the structural appearance of a transcription factor as well as motifs important in programmed cell death; it is expressed at high levels in fetal brain but not in spleen or thymus. We have demonstrated that ectopic ETO expression in NIH/3T3 cells leads to foci of transformation and colony growth in soft agar. Furthermore, ETO-transfected NIH/3T3 cells were tumorigenic in nude mice. Our data suggests that ETO may play a direct role in the leukemic transforming potential of the AML1/ETO fusion, in a manner additive to that of AML1. We have also recently discovered a novel interaction between the zinc finger domain of ETO and a new gene highly homologous to the nuclear receptor co-repressor gene. We are in the process of cloning and sequencing this new gene (about 9 kb cDNA) and determining its role in the transcriptional regulation of nuclear receptors such as the retinoic acid receptor. Understanding the interrelationship between the ETO oncoprotein and this nuclear receptor co-repressor should provide novel insights into the leukemogenic process.